turtl631

JZR, I know you used to do some open track stuff with an S2000. It seems that autocrossers use huge tires more than open track guys. Do you think these huge tires would be ideal on a track where rotational resistance is more of a factor and they have more time to cool off between corners? I remember reading some stuff about corvette guys turning faster times on the track with smaller wheel/tire combos than they use for the quickest autocross times. Just curious, thanks

jzr

Keeping tires in their ideal temperature range is important. For some very small and light cars it may be difficult to get tires fully up to temp, I've seen this on cars like the Lotus 7 (~1100lbs). With big tires and a really hard compound, a driver may not be able to get the tires to temp.

We played around a bit with tire sizes on the S2000, and the fastest setup was always the largest we had (slicks the equivalent of 285/325's) on a car weighing well under 2400 pounds. Took 1-2 hard laps to really get 'em sticky though.

As for the Corvette story, I would consider the source carefully. I would wager that in any serious (say Speed World Challenge or faster) endeavor, you're not likely to find anybody campaigning a production-based car running any tire smaller than either the maximum allowed by the rules, or the maximum size they can physically fit to the car.

turtl631

Hm, interesting. I guess this is yet another case where common sense/logic doesn't agree with experimental results. Thanks for the input, its always great to hear from people who actually know what they're talking about and have firsthand experience.

kapower

I auto X in the NWR SCCA. I race an s13 in SM, and competed against an s13 with 335mm r compounds all around. I believe they had 12" wheels x4. I have also raced against the PDM 240.

Don from pdm told me that +4 offset would be about perfect for a 10" wheel up front. But his 17x10 +12 5zygens with 275's up front had 2 5mm spacers stacked together.

so for a 10.5" wheel a good offset maybe...

+2 offset - .25" (6.35mm) = -4.3mm offset or -2.3mm minimuim

I think Don's 275/45/17's bulged out more then a 285/30/18 so there should be a little more clearance with the 18" wheel and tire combo.

kapower

Custom made 4 lug 5zygens for the S13 240sx are 17x9 + 20 offset.

The backspacing will vary from wheel to wheel. And clearance will change with tire size, and coil overs but...

-a 10.5 wheel is 1.5" wider (1.5 x .5 = .75) (.75" = 19.05mm)

-19.05mm - 20mm = 0.95mm offset. I think zero offset would be a safe bet, but you could always throw on a quality hub centric 5mm spacer if needed.

On the subject of tire heat. I could heat my rear tires, 225/50/15 V710's, up to 140+ degs in 1 lap the fronts would be about 115-120 deg f. The car had stock everthing sr20 redtop at 14-15psi, Camber 2.5 rear, 4.5 front, with a little toe in on the rear. The 240sx definetly needs 275mm+ for auto X but you'll need a tire warmer (dual driver

IMO a 285/30/18 would be a great tire for an s13 in most auto X conditions.

I think the 275/35/15 will be more affordable, lighter, and lower the car more, but the smaller diameter means less tread contact on the ground so you will have less grip coming out of the corners while accelerating... But may overall be faster...

sideview_180sx

those 5zigen's weren't custom by any means. but if you read up on those who had them, they experienced coilover rubbing issues. I had them and I rubbed up front on the coilover spring, then i took a 5mm spacer to it and it cured that problem.

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jmauld

Since this is relevant to what the original poster was asking about and might be some of the same issues he's going to run into, I'll post my question here.

I've got a set of 285/30/18s that I'm trying to get on my S14 at the moment. Rim size is a 18x9 with 35mm offset. Everything fits fine in the rear. No fender rolling or pulling is necessary.

The front I'm having difficulty with. With no spacers, the rims are hitting the springs on my shocks (bilstein inserts in lowered OEM housing). I can use a 25mm spacer to push the rims out enough to clear the spring, but then the tire hits the fender liners when turning.

I can use a crash bolt to push the top of the rim out some and use a 15mm spacer. However, that only allows me to get about -1.5 degree of camber out of my tein plates. Are there any other camber plates that give more adjustment then the tein plates? Besides slotting the mount holes.

My other option is to put the bilstein inserts into a non-lowered housing and lift the spring back over the tire.

JZR, what are you using to push the bottom of the strut further inboard, and does that screw withthe suspension geometry?


Any other suggestions?

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Jason
'95 240sx SE

jmauld

This tire is kind of interesting. I have a set of 15's that they would fit on, so I may give them a try.

My only concern is what they are going to do to the gearing. I've already been running a 225/50/15 which is smaller than the original tires. At most of the local autocrosses, I'm all over the rev limiter in second gear. The 275/35/15 is an even smaller diameter which would end up putting you into third gear. Of course there are some small parking lot courses that these tires would just kick butt on!

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Jason
'95 240sx SE

jzr

No reason to use the 275/35-15 on the rear except in the case of a relatively low-powered stock car. A turbo 240 is just going to smoke 'em in the lower gears, at least with stock gearing. If you could shoehorn them on the 6" wide stock wheels, could be good for the car in Stock class. There's lots of room in the back to run a tall and wide tire, and it's easy to make even more room with the only sacrifice being the car's overall width.

In the front room is much more limited by the fender seam above and to the rear of the tire. The shorter the tire the better, assuming you can keep the ABS from getting too screwed up. Definitely worth trying up there.

The two ways to get more inboard clearance in front are to increase the length of the strut ears and running smaller diameter springs (2.25" and 2" options are available). Increasing strut ear lengths may compromise things somewhat, but such is life in dealing with strut suspensions.

Rubbing, grinding, and reduced steering lock are facts of life with big front tires on these cars, and it's not limited to strut cars. The Corvette I drove in '05 with 315 front tires, you could only turn the front wheel 3/4 turn before the inside of the rims ground into the control arms. Sure made correcting the all-too-frequent oversteer issues more difficult!

jmauld

For whatever reason, I've got my heart set on DSP. So I'm stuck with a relatively low-powered stock car. (Dyno'd 156hp to the wheels)

However, I got the 18's to fit this weekend.

This week, I'm taking off my shocks and sending them back to Bilstein to be rebuilt. Hopefully they'll be back before the local test & tune. If so, I'll find out how low I can lower the car, and get rid of that slight lean that I have.

Rims: 18x9, +35mm offset. 20mm spacer on the front
Tires: 285/30/18 Hoosier A6s.

No fender mods have been made yet.

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Jason
'95 240sx SE

nlzmo400r

looks friggin sick, what kind of tires? Look like slicks

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nlzmo400r

oops, nvmd, reading pwns me. How does the 285 fit on the wheel? Any closer pictures of the actual tire on wheel

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jmauld

Is this what you're looking for? I think the tire fits on the rim pretty good. To be honest, I was expecting it to bulge out a little more.

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Jason
'95 240sx SE

nlzmo400r

yea, thats what I was talking about, it does fit really well. Makes me think it would want to stretch on a 10.5

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jzr

That fitment isn't *bad* per se, but it would be better with a wider wheel. You can see the tread edge is rounded a bit, a wider wheel would flatten out the tread surface somewhat. I'll try to get a pic of mine on an 11" in the next day or two...

You car looks great Jason! I think it could be a great choice for DSP, go kick some BMW ass!